Eccentric screw pumps are used for the delivery of media capable of flowing in a viscous state, in particular media which are highly abrasive. The eccentric screw pumps consist of a stator having a through-opening. The inner wall of the through-opening is in the form of a multiple-start thread and is formed by an elastomer. The elastomer is located in a tubular shell made of high-strength material, for example steel, in which case the inner contour of the shell is either cylindrically smooth or follows the thread contour of the through-bore at a constant radial distance. Rotating in the through-bore of the stator is a rotor, the number of helices of which is one less than the number of thread helices in the through-bore. The rotor is made of a strong material and has an especially high abrasion resistance.
In the case of an eccentric screw pump, the rotor is driven from outside via a motor and it delivers through the through-bore in interaction with the stator. During the rotation of the rotor, crescent-shaped or banana-shaped chambers, in the widest sense, are produced in interaction with the inner wall of the through-bore, and these chambers gradually pass through the stator during the rotation of the rotor.
Such arrangements may also be used as a motor if the liquid is forced through the arrangement at high pressure. The pressure of the liquid sets the rotor in rotation and mechanical energy can be tapped at the rotor. Use is made of this arrangement, for example, in subsurface drilling motors.
The production of the stators is comparatively simple. They are vulcanized via a mold core and in this way are given the complicated shape of the through-opening. On the other hand, the production of the rotors has hitherto been more difficult, these rotors hitherto being produced from the solid material by machining processes.
It is certainly known from DE-A-1 703 828 to forge the rotor from a tube. Rotors of this type are not sufficiently dimensionally stable in the axial direction at high driving forces or high pressures, as occur in subsurface drilling motors. The driving torque leads, inter alia, to the rotor becoming twisted on account of its helical form and being shortened in the process. The result is that the calculated pitch of the rotor no longer corresponds to the calculated thread pitch of the multiple-start thread in the stator and leakages occur, which lead to pressure losses and thus to power losses.
Another type of construction of a rotor has been disclosed by DE-A-195 01 514. The rotor is composed of a shell and a core element contained in the shell. The shell is produced from a cylindrical tube by cold working. In this case, a drawing tool is pulled through the cylindrical tube, as a result of which the tube is given the helical form required for the rotor. The core element is subsequently loosely inserted in the shell thus produced and is connected to the tube at both ends.
However, it has been found that the accuracy to size at the outside of the shell is not sufficient and the shell has to be subjected to a secondary treatment. In addition, the known rotor twists to a relatively high degree due to its lack of torsional strength. The torsion leads to a change in the thread pitch [lacuna] thus to a pitch error relative to the stator, a factor which in turn adversely affects the sealing relative to the stator.
Described in DE-D-18 16 462 is a rotor whose shell consists of a ceramic mass. A steel shaft likewise passes through the hollow shell, the intermediate space between the inside of the shell and the steel shaft being filled with a bonding agent.
Starting therefrom, the object of the invention is to provide a rotor for an eccentric screw pump or an eccentric screw motor, for example a subsurface motor, which can be produced from [sic] in a comparatively cost-effective manner and is torsionally stable. This object is achieved according to the invention by the rotor having the features of claim 1.
In the novel rotor, a core element which is encased by a shell is used. On its outside, the shell forms the thread-shaped structure, i.e. the helically running area. In this way, the shell can be produced by cold working in a relatively cost-effective non-cutting manufacturing process. Located in the interior of the shell is a core element which runs through the shell over the entire length of the latter and gives the shell the requisite axial stability.
In this way, rotors may also be produced from materials which, although they are ductile, are difficult to machine, such as high-grade steels, e.g.
V2A or V4A steels. On the other hand, the core element can be made of a lower-grade steel.
As a result of the helical form of the shell, this shell, under the effect of the torque, could theoretically change in length in the manner known from the prior art if it is twisted. The use of the core element prevents the shell from being axially shortened in this way.
The core element may be a simple body which is cylindrical on the outside and is very simple and inexpensive to produce.
Since the shell is forged onto the core element in the case of the rotor according to the invention, a very strong connection is produced between the core element and the shell. This strong connection improves the torsional strength and also helps to ensure that the length of the rotor virtually does not change to a significant degree even under loading.
The forming of the shell onto the core element also brings about the advantage that the surface of the rotor no longer has to be reworked. The forming gives it its final and smooth surface, which, moreover, is bright if the forming takes place by cold working.
At the same time, the cold working has the further favorable secondary effect that the pitch of the rotor does not change, as would be the case of a hot forging process were to be used. In the case of hot forging, the change in length occurring during the cooling would have to be taken into account in a short time ago [sic].
The entire structure can thus be produced by non-cutting shaping.
The shell mounted on the core element has essentially the same wall thickness over its entire length and its circumference, i.e. it is approximately of the same thickness at every point.
The core element is in contact with the shell only in sections. These sections are regions of the thread valleys of the shell. In the region between the thread valleys, that is to say [lacuna] the thread crests of the shell, there are intermediate spaces between the core element and the shell. These intermediate spaces have the form of a single-or multiple-start screw.
During the cold working of the shell, it is possible for the shaping to be carried out only to the extent that the thread valleys of the shell only just touch the core element. The connection between the core element and the shell is then virtually a frictional connection.
However, it is possible to have the cold working carried out to such an extent that the core element is also shaped or the wall thickness of the shell at the contact point with the core element changes slightly. The connection with the core element is then also a positive-locking connection to a certain degree in this region, and it can also become an integral connection as a result of cold welding.
An especially torsionally resistant connection between the core element and the shell is achieved if the core element, at least in one section of its longitudinal extent, contains at least one groove which has a different course from the thread valley. An appropriate position of this groove relative to the thread valley enables the shell to be forged into this groove of the core element during the manufacturing process. Since the direction of this groove differs from the course of the thread valley, this reliably prevents the shell from being unscrewed from the core element along the screw formed by the thread valley.
Especially effective locking is achieved if the core element has at least one groove which is continuous over its entire axial length. In this case, the production of the core element becomes very simple if this groove follows the generating line.
As viewed in the circumferential direction, the groove expediently has a width as corresponds approximately to the contact region between the inside of the shell in the region of the thread valley and the core element. The depth of the groove is between 0.1 to 1.5 mm, about 0.5 mm has proved to be expedient.
It is favorable if the core element has a plurality of grooves.
The rotor according to the invention may have wall thicknesses of between 2 and 20 mm at an overall diameter of between 30 and 300 mm. The length of the novel rotor may be up to 8 m.
In order to connect the coupling head to the rotor, the core element, at one end, has a stem projecting beyond the shell. This stem is expediently designed as a threaded stem.
The rotor according to the invention can be used in eccentric screw pumps or arrangements which are used as motors, for example subsurface drilling motors.
Apart from that, developments of the invention are the subject matter of subclaims.